Abnous, K., Dieni,
C.A. and
Storey, K.B. 2008. Regulation of Akt
during hibernation in Richardson’s
ground squirrels. Biochim. Biophys. Acta
1780,
185–193. DOI:10.1016/j.bbagen.2007.10.009
Regulation
of Akt during hibernation in Richardson’s
ground squirrels
Khalil
Abnous, Christopher A. Dieni and Kenneth B. Storey
Abstract
Akt (or protein kinase B) plays a central role in coordinating growth,
survival and anti-apoptotic responses in cells and we hypothesized that changes
in Akt activity and properties would aid the reprioritization of metabolic
functions that occurs during mammalian hibernation. Akt was analyzed in
skeletal muscle and liver of Richardson's
ground squirrels, Spermophilus richardsonii, comparing the enzyme from
euthermic and hibernating states. Akt activity, measured with
a synthetic peptide substrate, decreased by 60–65% in both organs during
hibernation. Western blotting showed that total Akt protein did not
change in hibernation but active, phosphorylated Akt (Ser 473) was reduced by
40% in muscle compared with euthermic controls and was almost undetectable in
liver. Kinetic analysis of muscle Akt showed that S0.5 values
for Akt peptide were 28% lower during hibernation, compared with the euthermic enzyme,
whereas S0.5 ATP increased by 330%. Assay at 10 °C also
elevated S0.5 ATP of euthermic Akt by 350%. Changes in ATP
affinity would limit Akt function in the hibernator since the muscle adenylate
pool size is also strongly suppressed during cold torpor. Other parameters of
euthermic and hibernator Akt were the same including activation energy
calculated from Arrhenius plots and sensitivity to urea denaturation. DEAE
Sephadex chromatography of muscle extracts revealed three peaks of Akt activity
in euthermia but only two during hibernation suggesting isozymes are
differentially dephosphorylated during torpor. Altered enzyme properties and
suppression of Akt activity would contribute to the coordinated suppression of
energy-expensive anabolic and growth processes that is needed to maintain
viability during over weeks of winter torpor.